This invention relates to apparatus for illuminating a distant object and for reading optical information contained on the distant object. More specifically, the invention relates to apparatus for reading information such as alpha-numeric information contained on a license plate of a vehicle.
A number of systems have been proposed for optically or electronically identifying motor vehicles in order to facilitate the flow of traffic in urban areas. Such a system can be used, for example, at an automatic toll station where a motorist simply drives through a designated lane and an electronic device reads and records his license plate number. This information can then be used to charge a toll to the motorist's account. Identification systems can also be used to control entry of vehicles to restricted areas and to detect vehicles which may have been stolen or used for illegal purposes. In the latter use, it is often desirable to use a system which is operable to obtain the information without detection by the motorist.
Various arrangements are known in the prior art for optically identifying vehicles. In U.S. Pat. No. 4,368,479, there is shown an automobile identification system for use with an automobile license plate containing alpha-numeric characters in combination with a hologram containing encoded information which cannot be decoded by the unaided eye. In reading the license plate, a measuring CO2 laser is used to irradiate the license plate, and the optical information in the reflected image is processed by a receiver to retrieve both the visible information and the information in the hologram. A problem with the patented apparatus is that a specially constructed license plate having a hologram must be used by a motorist for the system to work. A further problem is that the light source and receiving device are separate units which makes the system relatively complex and expensive.
Other systems are known for reading optical information on an object located at a distance from a recorder. The patent to Hansen, U.S. Pat. No. 5,359,185, discloses apparatus for reading a bar code over a substantial range of distances. The Hansen apparatus comprises an illumination source for illuminating the bar code with light and a CCD sensor array for converting the image information into an electrical signal. The illumination source is adapted to sequentially illuminate the bar code with light of varying wavelengths. A computer is used to decode images produced at the various wavelengths to find an image of sufficient focus. The elaborate ranging techniques disclosed in this patent are costly and are not needed in many applications, such as vehicle identification systems. A further disadvantage is that the apparatus uses visible light which makes it impractical for certain uses.